The subject application claims benefit of the earlier filing dates of Japanese Patent Application Nos. 2000-222982 and 2000-363644 filed on Jul. 24, 2000 and Nov. 29, 2000 under the Paris Convention, the entire contents of which are incorporated by reference herein.
1. Field of the Invention
The present invention relates to a semiconductor switching device for detecting and shutting down an over-current caused by current vibration. More particularly, the present invention relates to a semiconductor switching device capable of detecting an over-current even when a rush current supplies a load, such as a lamp load or a motor load.
2. Description of the Related Art
A conventional semiconductor switching device (power semiconductor device) is employed for power supply control. The semiconductor switching device selectively supplies a battery power to respective loads in an automobile. The semiconductor switching device controls the power supply to a load by an incorporated thermal FET.
The semiconductor switching device has a shunt resistor. One end of the shunt resistor is connected to a power supply for supplying an output voltage VB. Further, a drain terminal of the thermal FET is connected to the other end of the shunt resistor. A load is connected to a source terminal of the thermal FET and is applied to automobile headlights or power window driving motor and the like. The semiconductor switching device is provided with an A/D converter and a microcomputer (CPU) for controlling the ON/OFF state of a drive signal of the thermal FET based on a current value monitored by the shunt resistor. Further, the semiconductor switching device includes a driver for controlling the thermal FET, under the control of the A/D converter and microcomputer (CPU).
The thermal FET incorporates a power device (mainly the FET), a resistor, a temperature sensor, a latch circuit, and an overheat shutdown FET. The foregoing elements configure a gate shutdown circuit. The gate shutdown circuit provides an overheat shutdown function for controlling the thermal FET (power device) to be forcibly turned OFF when the junction temperature of the thermal FET rises to a predetermined temperature or higher. When the temperature sensor detects that the power device has a predetermined or higher temperature increase, the detection information is maintained in the latch circuit, the overheat shutdown FET of a gate shutdown circuit is turned ON, and the power device is controlled to be forcibly turned OFF.
However, a shunt resistor is connected in series to a power supply path in order to detect a current. In recent years, there is a problem that the thermal loss of the shunt resistor due to a large load current cannot be ignored.
In addition, the above described overheat shutdown function and the gate shutdown circuit functions when a substantially complete short-circuit state is generated by a load, and a large current flows. However, the overheat shutdown function does not operate in the case of a small short-circuit current, such as so called layer short, which includes an incomplete short circuit having a small degree of short-circuit resistance.
A current may be monitored, and an over-current is detected by means of a microcomputer, whereby the thermal FET can be controlled so as to be turned OFF. However, the microcomputer control may have problems in responding to such over-current.
In addition, a shunt resistor, A/D converter, and a microcomputer and the like are required. Thus, a large package space is required. Moreover, there is a problem that the equipment cost is increased by these comparatively expensive elements.
In the case where a lamp (such as a headlight) load is the load, when a voltage is applied to the lamp load, a rush or initial current flows that is about 10 times as large as than when the lamp load has been in use for a period of time. Conventionally, when a current is detected without being limited to the above method, the rush current is masked, and is not detected. In the case where the lamp load or the like is used, the rush current is masked, and is not detected. Thus, the detection of over-current is delayed, and the switching device or wire has been excessively heated.
It is an object of the present invention to provide a semiconductor switching device capable of detecting an over-current, even with a rush current flow, the semiconductor switching device and high speed shut down of an over-current, such as an incomplete short-current.
In order to achieve the foregoing object, according to a feature of the present invention, there is provided a semiconductor switching device including
a multi-source FET including a main electric field effect transistor (FET) and a reference FET;
a reference current setting circuit for feeding a reference current including a constant component current and a transient component current to the reference FET such that a source potential of the main FET is not lower than that of the reference FET when a load current flowing to a main FET is not within the range of an over-current containing a transient component;
a voltage comparator for detecting that the source potential of the main FET is lower than that of the reference FET;
a counter for counting the number of variations of the reference voltage to a predetermined number of vibrations; and
a gate driving circuit for turning OFF the main FET by counting the counter. This makes it possible to shut down a current without the presence or absence of a transient component if a current exceeding a normal range flows a current on the load side.
Other and further objects and features of the present invention will become obvious upon an understanding of the illustrative embodiments about to be described in connection with the accompanying drawings or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employing of the invention in practice.